1. Field of the Invention
This invention relates to an optical network component, and in particular to an optical network comprising one or more of such components.
2. Related Art
Currently, in the United Kingdom, the telecommunications network includes a trunk network which is substantially completely constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. In future, it would be highly desirable to have a fixed, resilient, transparent telecommunications infrastructure all the way to customer premises, with capacity for all foreseeable service requirements. One way of achieving this would be to create a fully-managed fibre network for the whole access topography. Preferably, such a topography should comprise passive optical networks (PONs) which incorporate single mode optical fibre and no bandwidth-limiting active electronics.
In a PON, a single fibre is fed out from a head-end (exchange), and is fanned out via passive optical splitters at cabinets and distribution points (DPs) to optical network units (ONUs). The ONUs can be in customers' premises, or in the street serving a number of customers. The use of optical splitters enables sharing of the feeder fibre and the exchange-based optical line termination (OLT) equipment, thereby giving PONs cost advantages. At present, simplex deployment of PONs is the preferred option, that is to say separate upstream and downstream PONs are provided whereby each customer has two fibres. A downstream PON (that is to say a PON in which traffic is transmitted by the head-end for reception by the customers) uses passive optical splitters to feed the ONUs. Similarly, an upstream PON (that is to say a PON in which traffic is transmitted to the head-end by the customers) uses passive optical combiners to combine the customer traffic for reception by the head-end. In practice, the splitters and combiners are identical in structure. Although simplex working increases the complexity of the infrastructure due to the two fibres per circuit required, it benefits from a low optical insertion loss (owing to the absence of duplexing couplers), and a low return loss, since such systems are insensitive to reflections of less than 25 dBm with separate transmit and receive paths. Typically, a PON has a four-way split followed by an eight-way split, so that a single head-end fibre can serve up to 32 customers.
In a known arrangement--TPON (telephony over a passive optical network)--a head-end station broadcasts time division multiplex (TDM) frames to all the terminations on the network. The transmitted frames include both traffic data and control data. Each termination recognises and responds to appropriately-addressed portions of the data in the broadcast frames, and ignores the remainder of the frames. In the upstream direction, each termination transmits data in a predetermined timeslot, and the data from the different terminations are assembled at the head-end into a time division multiple access (TDMA) frame of predetermined format.
The present applicant has developed a bit transport system (BTS) for use in a PON which operates using TDMA. The BTS is described in our European patent specifications 318331, 318332, 318333 and 318335.
Recently, the PON principle has been expanded to form what is known as the SuperPON concept, in which high power optical amplifiers are used to allow very large, high split PONs to be built. For example, the use of optical amplifiers (such as fibre amplifiers) permits up to 3500 customers to be connected to a single head-end station over distances of up to 200 km.
Unfortunately, until now, optical amplifiers have only been used on downstream SuperPONs, as the use of amplifiers on an upstream SuperPON would cause noise problems resulting from the superposition of amplified stimulated emissions (ASEs) from the amplifiers.
An example of an optical communications network for broadcasting television signals and also for providing bidirectional voice and data communications is disclosed in European patent application 0 499 065. Optical amplifiers are provided in both the upstream and downstream directions. Anoptical fibre telecommunications line which has one or more gated optical amplifiers is disclosed in European patent application 0 506 163. The gated optical amplifiers are operable to produce substantially no output if their input is below a predetermined threshold. In this way, on the line being broken upstream of the amplifier, the amplifier produces substantially no output causing the link to shut down and thereby preventing light escaping from the broken fibre from injuring maintenance personnel.